Method for deriving precise control over laser power of an optical pickup unit, and associated automatic power calibration circuit

ABSTRACT

A method for deriving precise control over laser power of an optical pickup unit (OPU) includes: providing an analog-to-digital converter (ADC) within an automatic power calibration (APC) circuit to derive a path gain and/or a path offset from the APC circuit; and selectively performing compensation according to the gain and/or the path offset, in order to maintain precision of a relationship between the laser power and a target command utilized for controlling the laser power. An associated APC circuit comprising an ADC and at least one compensation module is further provided. The ADC is utilized for deriving a path gain and/or a path offset from the APC circuit. The compensation module is utilized for selectively performing compensation according to the path gain and/or the path offset, in order to control the laser power by a target command.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application and claims the benefit ofU.S. application Ser. No. 12/195,414, which was filed on Aug. 21, 2008,and entitled “METHOD FOR DERIVING PRECISE CONTROL OVER LASER POWER OF ANOPTICAL PICKUP UNIT, AND ASSOCIATED AUTOMATIC POWER CALIBRATIONCIRCUIT”. In addition, the aforementioned U.S. application Ser. No.12/195,414 claims the benefit of U.S. Provisional Application No.60/991,185, which was filed on Nov. 29, 2007, and entitled “AUTO POWERCALIBRATION STRUCTURAL”.

BACKGROUND

The present invention relates to power calibration of an optical pickupunit (OPU) with respect to a target command during a mass productionphase of an optical disc drive, and more particularly, to a method forderiving precise control over laser power of an OPU, and to anassociated automatic power calibration (APC) circuit.

Regarding the control over an OPU of an optical disc drive in therelated art, a conventional APC circuit can be utilized for controllingthe laser power of a laser diode (LD) during a normal operation of theoptical disc drive, e.g. a reading/writing operation. When theconventional APC circuit reaches a steady state during the normaloperation mentioned above, the laser power corresponds to a targetcommand sent to the conventional APC circuit. It is a goal for theconventional APC circuit to control the laser power to be a specificpower value corresponding to the target command, in order that the laserpower varies in accordance with the target command. Sometimes, the goalmentioned above appears to be too idealized to achieve, the reason forwhich is described as follows.

A conventional method for deriving the relationship between the laserpower and the target command typically comprises measuring the laserpower by utilizing a power meter, and collecting data sets of the laserpower and the target command. However, the cost of the power meter ishigh, and the corresponding tooling and labor costs of a powercalibration station for implementing this method are also required.Additionally, another issue such as the differences between respectivepower calibration stations may arise.

According to the related art, an OPU vendor may design a front-end photodiode (PD) in an OPU, and the manufacturers (e.g. an optical disc drivemanufacturer) uses the front-end PD as a replacement for the powermeter. The measurement result from the front-end PD is outputted througha front-end PD output (FPDO), and can be referred to as the FPDO value.Some examples of curves of a relationship between the laser power of theOPU and the FPDO value are illustrated in FIG. 1. As shown in FIG. 1,the curve passing through the origin corresponds to an ideal case, andthe other two curves correspond to a real case with a positive offsetand another real case with a negative offset.

As the OPU vendor typically provides a few data points for stating therelationship between the laser power and the FPDO value, interpolationoperations are required for deriving the laser power corresponding toother data points on a predicted curve passing through the few datapoints mentioned above. As a result, the whole process of deriving aprecise relationship between the laser power and the target command isslowed down due to the interpolation operations.

In addition, when trying to derive the relationship as mentioned withthe laser power having a duty cycle such as 50% (e.g. the 50% duty cyclewrite power), it is very hard to accurately measure a direct current(DC) component by utilizing the FPDO value due to various hardwarelimitations, e.g. analog bandwidth.

Additionally, the gain and the offset of the conventional APC circuitmay vary from chip to chip, and the overall gain and the overall offsetof the combination of the OPU and the conventional APC circuit may alsovary from system to system. Thus, when using the FPDO, the plan ofderiving a precise relationship between the laser power and the targetcommand does not work well in practice.

SUMMARY

It is therefore an objective of the claimed invention to provide amethod for deriving precise control over laser power of an opticalpickup unit (OPU), and to provide an associated automatic powercalibration (APC) circuit, in order to solve the above-mentionedproblems.

It is another objective of the claimed invention to provide a method forderiving precise control over laser power of an OPU, and to provide anassociated APC circuit, in order to derive a precise relationshipbetween the laser power and a target command for controlling the laserpower.

An exemplary embodiment of a method for deriving precise control overlaser power of an OPU comprises: providing an analog-to-digitalconverter (ADC) within an APC circuit to derive a path gain and/or apath offset from the APC circuit; and selectively performingcompensation according to the gain and/or the path offset, in order tocontrol the laser power by a target command. In particular, the step ofselectively performing compensation according to the path gain and/orthe path offset further comprises: compensating the path gain when thepath gain is derived; and canceling the path offset when the path offsetis derived.

An exemplary embodiment of an APC circuit for controlling laser power ofan OPU comprises an ADC and at least one compensation module that iscoupled to the ADC. The ADC is utilized for performing analog-to-digitalconversion, and further utilized for deriving a path gain and/or a pathoffset from the APC circuit. The compensation module is utilized forselectively performing compensation according to the path gain and/orthe path offset, in order to maintain precision of a relationshipbetween the laser power and a target command utilized for controllingthe laser power. In particular, the APC circuit further comprises acontrol unit arranged to control the APC circuit, wherein the controlunit controls the APC circuit to compensate the path gain when the pathgain is derived, and controls the APC circuit to cancel the path offsetwhen the path offset is derived.

An exemplary embodiment of a method for deriving precise control overlaser power of an OPU comprises: providing an ADC within an APC circuitto derive a path offset from the APC circuit; and selectively performingcompensation according to the path offset, in order to control the laserpower by a target command.

An exemplary embodiment of an APC circuit for controlling laser power ofan OPU comprises an ADC and at least one compensation module that iscoupled to the ADC. The ADC is utilized for performing analog-to-digitalconversion, and further utilized for deriving a path offset from the APCcircuit. The compensation module is utilized for selectively performingcompensation according to the path offset, in order to maintainprecision of a relationship between the laser power and a target commandutilized for controlling the laser power.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates curves of a relationship between laser power of anoptical pickup unit (OPU) and a front-end PD output (FPDO) valueoutputted from an FPDO of an OPU according to the related art.

FIG. 2 is a diagram of an automatic power calibration (APC) circuit forcontrolling laser power of an OPU according to a first embodiment of thepresent invention.

FIG. 3 is a diagram of an APC circuit for controlling laser power of anOPU according to a second embodiment of the present invention.

DETAILED DESCRIPTION

Certain terms are used throughout the following description and claims,which refer to particular components. As one skilled in the art willappreciate, electronic equipment manufacturers may refer to a componentby different names. This document does not intend to distinguish betweencomponents that differ in name but not in function. In the followingdescription and in the claims, the terms “include” and “comprise” areused in an open-ended fashion, and thus should be interpreted to mean“include, but not limited to . . . ”. Also, the term “couple” isintended to mean either an indirect or direct electrical connection.Accordingly, if one device is coupled to another device, that connectionmay be through a direct electrical connection, or through an indirectelectrical connection via other devices and connections.

Please refer to FIG. 2. FIG. 2 is a diagram of an automatic powercalibration (APC) circuit 100 for controlling laser power of an opticalpickup unit (OPU) 50 according to a first embodiment of the presentinvention. The OPU 50 is typically positioned in an optical disc drivesuch as a compact disc-read only memory (CD-ROM) drive, a digitalversatile disc (DVD) drive, HD-DVD drive, etc., where the OPU 50 is acomponent that is well known in the related art.

In addition to the APC circuit 100 mentioned above, the presentinvention further provides a method for deriving precise control overlaser power of an OPU. The method can be applied to the APC circuit 100and implemented by utilizing the APC circuit 100, and is described asfollows.

As shown in FIG. 2, the APC circuit 100 comprises an analog-to-digitalconverter (ADC) 112, an analog gain amplifier 114 and a multiplexer 116(labeled “MUX”), and further comprises at least one compensation modulesuch as a compensation module 120. In this embodiment, the compensationmodule 120 comprises a target command input unit 122 and an arithmeticunit 124.

According to this embodiment, the ADC is utilized for performinganalog-to-digital conversion, and further utilized for deriving a pathgain and/or a path offset from the APC circuit 100. Please note that thepath offset mentioned above typically represents an overall offset dueto the OPU 50 and the APC circuit 100. In practice, the APC circuit 100can be implemented with a chip mounted on a printed circuit board (PCB),and therefore, the path offset represents an overall offset due to theOPU 50, the chip and the PCB.

In addition, the compensation module 120 cooperated with the multiplexer116 is utilized for “selectively” performing compensation according tothe path gain and/or the path offset, in order to maintain precision ofa relationship between the laser power and a target command utilized forcontrolling the laser power, where the meaning of the adverb“selectively” will be explained later.

According to this embodiment, the path gain and/or the path offset arederived from the APC circuit 100 to complete a configuration of the APCcircuit 100 when designing/manufacturing an electronic device comprisingthe OPU 50 and the APC circuit 100. In this embodiment, the electronicdevice mentioned above may represent an optical disc drive such as thatmentioned above, represent a video playback device comprising adisc-accessing module capable of reading an optical disc such as aCD-ROM, DVD, HD-DVD, etc., or represent a digital video recorder (DVR)comprising a disc-accessing module capable of accessing an optical disc.

The APC circuit 100 of this embodiment further comprises a control unit(not shown) arranged to control the APC circuit 100, where variousimplementation choices can be applied. For example, the control unit canbe a micro-processing unit (MPU) executing a program code such as afirmware code for realizing a complete operation of the APC circuit 100,where the MPU is typically implemented in an individual chip or in thesame architecture (e.g. the same chip) with those components of the APCcircuit 100 shown in FIG. 2. In another example, the control unit can bea dedicated hardware controller executing a hardware code for realizingthe complete operation of the APC circuit 100, where the dedicatedhardware controller is typically implemented in the same architecture(e.g. the same chip) with those components of the APC circuit 100 shownin FIG. 2.

No matter which implementation choice is applied to the control unitmentioned above, the control unit controls the APC circuit 100 tocompensate the path gain when the path gain is derived, and controls theAPC circuit 100 to cancel the path offset when the path offset isderived. In most of the cases that are encountered, the path gaindeviates from a design gain value and the path offset is non-zero. Thus,compensating the path gain and canceling the path offset are bothrequired. Only in a very few cases, the path gain is substantially equalto the design gain value or the path offset is zero. In general, thecompensation module 120 “selectively” performs compensation according tothe path gain and/or the path offset.

In this embodiment, the analog gain amplifier 114 is arranged to receivea first voltage level and generate a second voltage level accordingly,and is utilized for applying a specific gain value to the first voltagelevel. In an ideal case, the specific gain value is substantially adesign gain value such as that mentioned above. In most of the casesthat are encountered, the specific gain value deviates from the designgain value.

In addition, the multiplexer 116 is arranged to multiplex the firstvoltage level or the second voltage level according to the control ofthe control unit. When the multiplexer 116 multiplexes the first voltagelevel, the ADC 112 converts the first voltage level into a first value.When the multiplexer 116 multiplexes the second voltage level, the ADC112 converts the second voltage level into a second value. The controlunit mentioned above thus calculates a ratio of the second value to thefirst value to derive the path gain.

According to the method mentioned above, various configurationconditions (i.e. conditions for configuring the APC circuit 100) can beapplied to the APC circuit 100 to derive the path gain and the pathoffset.

The configuration conditions for deriving the path gain are described asfollows. For example, in one of the configuration conditions, an inputterminal 113 for receiving the first voltage level is temporarilyisolated from the OPU 50 (i.e. an open loop is formed regarding the APCcircuit 100 and the OPU 50). Thus, the path gain can be derived byapplying a specific voltage level as the first voltage level. In a powercalibration station for implementing this configuration condition, thespecific voltage level can be outputted from a power supply. As aresult, the path gain is substantially equal to the ratio of the secondvalue to the first value.

According to this configuration condition, the specific voltage level isoutputted from the power supply of inexpensive price as mentioned. Thisis for illustrative purposes only, and is not meant to be a limitationof the present invention. According to another configuration conditionfor deriving the path gain, the specific voltage level can be derivedfrom a specific component within the aforementioned chip forimplementing the APC circuit 100. For example, the specific componentcan be a level shifter. In another example, the specific component canbe a regulator. Similar descriptions for this configuration conditionare not repeated in detail here.

Regarding another configuration condition, the input terminal 113 of theanalog gain amplifier 114 is temporarily coupled to a front-end PDoutput (FPDO) of a front-end PD 54 within the OPU 50 as shown in FIG. 2(i.e. a closed loop is formed by the APC circuit 100 and the OPU 50).Thus, the path gain can be derived by applying a specific value as thetarget command. In this configuration condition, the specific valueshould enable a laser diode (LD) 52 of the OPU 50 to emit a laser. Inorder to prevent interference due to noise, it would be better that thelaser power is not too small.

In addition, the second value is derived after the APC circuit 100enters a second steady state with the second voltage level (generatedfrom the analog gain amplifier 114) being multiplexed by the multiplexer116, and the first value can be derived just after the second value isderived. Additionally, the first value is derived after the APC circuit100 enters a first steady state with the first voltage level beingmultiplexed by the multiplexer 116. As a result, the path gain issubstantially equal to the ratio of the second value (which is derivedin the second steady state in this configuration condition) to the firstvalue (which is derived in the first steady state when the multiplexer116 switches as mentioned in this configuration condition).

According to this configuration condition, the first value is derivedjust after the second value is derived. This is for illustrativepurposes only, and is not meant to be a limitation of the presentinvention. According to another configuration condition for deriving thepath gain, the order for deriving the first value and the second valueis changed. For example, the second value can be derived just after thefirst value is derived. Similar descriptions for this configurationcondition are not repeated in detail here.

The configuration conditions for deriving the path offset are furtherdescribed as follows. For example, in one of the configurationconditions, a driving terminal 51 of the LD 52 is temporarily isolatedfrom the APC circuit 100 (i.e. an open loop is formed regarding the APCcircuit 100 and the OPU 50). Thus, a driving signal can be applied tothe driving terminal 51, where the driving signal has a voltage levelthat is close to or substantially equal to a ground level. In thisconfiguration condition, the input terminal 113 is temporarily coupledto the FPDO of the OPU 50, and the multiplexer 116 multiplexes thesecond voltage level (generated from the analog gain amplifier 114). Asa result, the ADC 112 converts a derivative of the second voltage level(i.e. the second voltage level multiplexed by the multiplexer 116 inthis configuration condition) into a third value to derive the pathoffset. More specifically, the path offset is substantially equal to thethird value.

Regarding another configuration condition, the input terminal 113 istemporarily coupled to the FPDO of the OPU 50 as shown in FIG. 2 (i.e. aclosed loop is formed by the APC circuit 100 and the OPU 50). Thus,another specific value can be applied as the target command to derivethe path offset. In this condition to derive the path offset, thespecific value should disable the LD 52 (e.g. the specific value causesa voltage level of a driving signal of the LD 52 to be lower than 0.7volts), so the LD 52 does not emit the laser. In addition, themultiplexer 116 multiplexes the second voltage level. As a result, theADC 112 converts a derivative of the second voltage level (i.e. thesecond voltage level multiplexed by the multiplexer 116 in thisconfiguration condition) into the third value such as that mentionedabove to derive the path offset. More specifically, the path offset issubstantially equal to the third value after the APC circuit 100 entersa third steady state.

After deriving the path gain and/or the path offset as mentioned in oneor more configuration conditions, the control unit mentioned abovestores the path gain and/or the path offset for being utilized in anormal APC operation of the APC circuit 100. For example, the controlunit may store the path gain and the path offset into a non-volatilememory. After the configuration of the APC circuit 100 is completed, thenormal APC operation can be performed for precisely controlling thelaser power.

In the normal APC operation, a compensated target value can becalculated and dynamically adjusted in real time according to the pathgain and the path offset, and is expressed as follows:TC_(A)(t)=FPDO(t)*PG−PO;where TC_(A)(t) represents the compensated target value in analog type,FPDO(t) represents the FPDO value derived online, PG and PO respectivelyrepresent the path gain and the path offset, and t is an indexcorresponding to time.

According to this embodiment, the target command input unit 122 isutilized for inputting the target command, and the arithmetic unit 124is utilized for performing a subtraction operation according to a targetcommand representative (i.e. a representative of the target command)outputted from the target command input unit 122 and the voltage leveloutputted from the multiplexer 116. For example, the arithmetic unit 124subtracts the target command representative from the voltage level.

Therefore, in the normal APC operation, the control unit mentioned abovedynamically applies the compensated target value TC_(A)(t) as the targetcommand, and inputs the target command into the target command inputunit 122. The analog low pass filter (LPF) 150 shown in FIG. 2 is acomponent well known in the related art, and therefore, not explained indetail here.

FIG. 3 is a diagram of an APC circuit 200 for controlling laser power ofan OPU such as the OPU 50 according to a second embodiment of thepresent invention. This embodiment is a variation of the firstembodiment, where some of the components shown in FIG. 2 are replaced.As shown in FIG. 3, in addition to the ADC 112, the analog gainamplifier 114 and the multiplexer 116, the APC circuit 200 furthercomprises a plurality of compensation modules 220 and 230. According tothis embodiment, a digital offset cancellation module 220C of thecompensation module 220 (i.e. a digital offset input unit 222 and anarithmetic unit 224) and the compensation module 230 are respectivelyimplemented with a digital version of the compensation module 120. Inaddition to the digital offset input unit 222 and the arithmetic unit224, the compensation module 220 further comprises a digital controller226.

In the normal APC operation, the control unit mentioned above appliesthe path offset to the digital offset cancellation module 220C throughthe digital offset input unit 222, so the digital offset cancellationmodule 220C digitally cancels the path offset by utilizing thearithmetic unit 224. More specifically, the digital offset cancellationmodule 220C performs an arithmetic operation by utilizing the arithmeticunit 224 to cancel the path offset, where the arithmetic operation isperformed with digital value calculations. Here, the arithmeticoperation can be achieved by subtracting the path offset from a digitalvalue outputted by the ADC 112 to generate a digital value as the outputof the digital offset cancellation module 220C.

In addition, the digital controller 226 digitally compensates the pathgain. More specifically, the digital controller 226 performs acompensation operation with digital value calculations to compensate thepath gain. Here, the compensation operation can be achieved bymultiplying a digital value from the arithmetic unit 224 by the pathgain to generate a digital value as the output of the digital controller226. Thus, the control unit mentioned above applies a compensated targetvalue as the target command, where the compensated target value can beexpressed as follows:TC_(D)(t)=FPDO(t)*DG;where TC_(D)(t) represents the compensated target value in digital type,FPDO(t) represents the FPDO value derived online, DG represents thedesign gain value, and t is still an index corresponding to time.

Therefore, in the normal APC operation, the control unit mentioned abovedynamically applies the compensated target value TC_(D)(t) as the targetcommand, and inputs the target command into the target command inputunit 232. The arithmetic unit 234 is similar to the arithmetic unit 224.The digital LPF 250 shown in FIG. 3 is a component well known in therelated art, and therefore not explained in detail here.

In contrast to the related art, the present invention method andassociated APC circuit can save the time and related costs required fora power calibration station such as that mentioned above.

It is another advantage of the claimed invention that the presentinvention method and associated APC circuit can derive a preciserelationship between the laser power and a target command forcontrolling the laser power without utilizing a power meter. Thus,related costs can be saved.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

1. A method for deriving precise control over laser power of an opticalpickup unit (OPU), the method comprising: providing an analog-to-digitalconverter (ADC) within an automatic power calibration (APC) circuit toderive a path gain and/or a path offset from the APC circuit; andselectively performing compensation according to the path gain and/orthe path offset, in order to control the laser power by a targetcommand, wherein the step of selectively performing compensationaccording to the path gain and/or the path offset further comprises:compensating the path gain when the path gain is derived; and cancelingthe path offset when the path offset is derived; wherein the OPUcomprises a front-end PD output (FPDO) for outputting an FPDO value; andthe method further comprises: applying a compensated target value as thetarget command, wherein the compensated target value is substantiallyequal to a product of the FPDO value and the path gain minus the pathoffset.
 2. The method of claim 1, wherein the APC circuit comprises ananalog gain amplifier for generating a second voltage level according toa first voltage level; the analog gain amplifier has an input terminalfor receiving the first voltage level; and the method further comprises:applying a driving signal to a driving terminal of a laser diode (LD) ofthe OPU with the driving terminal being temporarily isolated from theAPC circuit, wherein the driving signal has a voltage level that isclose to or substantially equal to a ground level; and with the inputterminal being temporarily coupled to the FPDO of the OPU, convertingthe second voltage level or a derivative thereof into a third value byutilizing the ADC to derive the path offset.
 3. The method of claim 1,wherein the APC circuit comprises an analog gain amplifier forgenerating a second voltage level according to a first voltage level;the analog gain amplifier has an input terminal for receiving the firstvoltage level; and the method further comprises: with the input terminalbeing temporarily coupled to the FPDO of the OPU, applying a specificvalue and converting the second voltage level or a derivative thereofinto a third value by utilizing the ADC to derive the path offset;wherein the path offset is substantially equal to the third value afterthe APC circuit enters one of a plurality of steady states thereof. 4.The method of claim 1, further comprising: performing an arithmeticoperation with digital value calculations to cancel the path offset; andperforming a compensation operation with digital value calculations tocompensate the path gain; wherein the compensated target value issubstantially equal to a product of the FPDO value and a design gainvalue of an analog gain amplifier of the APC circuit.
 5. A method forderiving precise control over laser power of an optical pickup unit(OPU), the method comprising: providing an analog-to-digital converter(ADC) within an automatic power calibration (APC) circuit to derive apath gain and/or a path offset from the APC circuit; and selectivelyperforming compensation according to the path gain and/or the pathoffset, in order to control the laser power by a target command, whereinthe step of selectively performing compensation according to the pathgain and/or the path offset further comprises: compensating the pathgain when the path gain is derived; and canceling the path offset whenthe path offset is derived; wherein the APC circuit comprises an analoggain amplifier for generating a second voltage level according to afirst voltage level; and the method further comprises: multiplexing thefirst voltage level; converting the first voltage level into a firstvalue by utilizing the ADC; multiplexing the second voltage level;converting the second voltage level into a second value by utilizing theADC; and calculating a ratio of the second value to the first value inorder to derive the path gain.
 6. The method of claim 5, wherein theanalog gain amplifier has an input terminal for receiving the firstvoltage level; and the method further comprises: applying a specificvoltage level as the first voltage level with the input terminal beingtemporarily isolated from the OPU.
 7. The method of claim 5, wherein theanalog gain amplifier has an input terminal for receiving the firstvoltage level; and the method further comprises: applying a specificvalue as the target command with the input terminal being temporarilycoupled to a front-end PD output (FPDO) of the OPU; wherein the secondvalue is derived after the APC circuit enters a second steady state withthe second voltage level being multiplexed, and the first value isderived after the APC circuit enters a first steady state with the firstvoltage level being multiplexed.
 8. An automatic power calibration (APC)circuit for controlling laser power of an optical pickup unit (OPU), theAPC circuit comprising: an analog-to-digital converter (ADC) forperforming analog-to-digital conversion, wherein the ADC is utilized forderiving a path gain and/or a path offset from the APC circuit; at leastone compensation module, coupled to the ADC, for selectively performingcompensation according to the path gain and/or the path offset, in orderto maintain precision of a relationship between the laser power and atarget command utilized for controlling the laser power; and a controlunit arranged to control the APC circuit, wherein the control unitcontrols the APC circuit to compensate the path gain when the path gainis derived, and controls the APC circuit to cancel the path offset whenthe path offset is derived; wherein the OPU comprises a front-end PDoutput (FPDO) for outputting an FPDO value; and the control unit appliesa compensated target value as the target command, wherein thecompensated target value is substantially equal to a product of the FPDOvalue and the path gain minus the path offset.
 9. The APC circuit ofclaim 8, wherein a driving signal is applied to a driving terminal of alaser diode (LD) of the OPU with the driving terminal being temporarilyisolated from the APC circuit, and the driving signal has a voltagelevel that is close to or substantially equal to a ground level; and theAPC circuit further comprises: an analog gain amplifier arranged togenerate a second voltage level according to a first voltage level,wherein the analog gain amplifier has an input terminal for receivingthe first voltage level; wherein in a situation where the input terminalis temporarily coupled to the FPDO of the OPU, the ADC converts thesecond voltage level or a derivative thereof into a third value toderive the path offset.
 10. The APC circuit of claim 9, wherein the pathoffset is substantially equal to the third value after the APC circuitenters one of a plurality of steady states.
 11. The APC circuit of claim8, wherein the APC circuit further comprises: a digital offsetcancellation module arranged to perform an arithmetic operation withdigital value calculations to cancel the path offset; and a digitalcontroller arranged to perform a compensation operation with digitalvalue calculations to compensate the path gain; wherein the compensatedtarget value is substantially equal to a product of the FPDO value and adesign gain value of an analog gain amplifier of the APC circuit.
 12. Anautomatic power calibration (APC) circuit for controlling laser power ofan optical pickup unit (OPU), the APC circuit comprising: ananalog-to-digital converter (ADC) for performing analog-to-digitalconversion, wherein the ADC is utilized for deriving a path gain and/ora path offset from the APC circuit; at least one compensation module,coupled to the ADC, for selectively performing compensation according tothe path gain and/of the path offset, in order to maintain precision ofa relationship between the laser power and a target command utilized forcontrolling the laser power; a control unit arranged to control the APCcircuit, wherein the control unit controls the APC circuit to compensatethe path gain when the path gain is derived, and controls the APCcircuit to cancel the path offset when the path offset is derived; ananalog gain amplifier arranged to generate a second voltage levelaccording to a first voltage level; and a multiplexer arranged tomultiplex the first voltage level or multiplex the second voltage level;wherein the ADC converts the first voltage level into a first value andfurther converts the second voltage level into a second value; and aratio of the second value to the first value is calculated in order toderive the path gain.
 13. The APC circuit of claim 12, wherein theanalog gain amplifier has an input terminal for receiving the firstvoltage level; and the path gain is derived by applying a specificvoltage level as the first voltage level with the input terminal beingtemporarily isolated from the OPU.
 14. The APC circuit of claim 12,wherein the analog gain amplifier has an input terminal for receivingthe first voltage level; the path gain is derived by applying a specificvalue as the target command with the input terminal being temporarilycoupled to a front-end PD output (FPDO) of the OPU; and the second valueis derived after the APC circuit enters a second steady state with thesecond voltage level being multiplexed by the multiplexer, and the firstvalue is derived after the APC circuit enters a first steady state withthe first voltage level being multiplexed by the multiplexer.